In recent years, under the increase of the environmental consciousness, the soil pollution problems by disposal of plastic products and the global warming issue by increase of carbon dioxide caused by incineration is drawing attention. As a measure for the former, various biodegradable resins, and as a measure for the latter, resins made from plant-derived raw materials which do not give a new load of carbon dioxide to the air even when incinerated, are being actively researched and developed. As to shape-retaining devices such as used for display-wrapping of various products, or as to containers such as food trays or drink cup, those in which various biodegradable resins or plant-derived raw materials are used have been developed. Among them, in particular, polylactic acid which is an aliphatic polyester is most drawing attention since it has a high glass transition temperature and is transparent among the biodegradable or plant-derived plastics and the like, as a material of its future is big.
However, polylactic acid is low in glass transition temperature by approximately 20° C. compared to conventional petroleum-derived raw materials, for example, polyethylene terephthalate, and when it is applied to various conventional uses, there arises a problem that its heat resistance is insufficient.
As means for solving this problem, a technique of crystallizing polylactic acid to improve heat resistance is proposed.
For example, in JP-H9-278991 A, a technique in which a polylactic acid composition containing a transparent nucleating agent is heat treated when it is formed or after the forming, to increase crystallinity is described. In this technique, since heat resistance is imparted by crystallization by heat treatment in a forming mold when it is formed or by a heat treatment after the forming, the heat resistance of sheet itself was insufficient.
In JP 2003-245971 A, a technique in which, by heat treating the sheet before forming or by imparting orientation by drawing, releasability from a heated mold at the forming is imparted is described. However, in this technique, transparencies of the sheet and the formed article were greatly inferior.
In JP 2004-204128 A, a technique of imparting, by mixing a crystalline polylactic acid and an amorphous polylactic acid, a heat formability to a drawn sheet is described. However, in this technique, since the amorphous polylactic acid is contained more than 50%, a sufficient heat resistance can not be imparted.
In JP 2005-119062 A, a polylactic acid-based sheet of which folding endurance is high is proposed, by making into 2 kinds-3 layer sheet constitution of polylactic acid of which inner layer contains polylactic acid and a lactic acid-based polyester and outer layer contains a transparent nucleating agent. However, in the technique disclosed here, there was a problem that transparency becomes poor due to the heat treatment at the time of forming.
In JP 2005-125765 A, a technique in which formability and heat resistance are made compatible by heat crystallizing a laminate sheet comprising 2 layers different in containing amount of D-form of polylactic acid resin, that is, different in crystallinity, but in this technique, transparency was greatly insufficient.
It could thus be helpful to provide, in view of such a background, an aliphatic polyester-based sheet excellent in heat resistance, impact resistance, transparency and formability, and a formed object made thereof.